This invention relates to fluids or muds useful in the drilling of subterranean wells. More particularly, it relates to improved aqueous drilling mud additives comprising sulfonated low molecular weight copolymers of vinyl toluene and maleic anhydride which are useful as dispersants providing muds with improved colliodal stability under high temperature and/or high pressure conditions.
In drilling subterranean wells and especially when employing rotary methods, drilling muds or fluids perform a variety of functions which influence such factors as the drilling rate, cost, efficiency and safety of the operation. Drilling muds are circulated down through the drill shaft, out the bottom of the drill bit and up the annular space between the drill pipe and the formation wall to the surface where they may be mechanically or chemically treated and recirculated. The muds provide lubrication at the point where the drill bit cuts into a geologic formation as well as along the length of the rotary drill pipe. As it lubricates the system, the mud removes heat which is produced by the cutting action of the bit while additionally performing the important task of transporting cuttings produced by the system to the surface for removal. An acceptable mud must have body yet be free flowing with a measurable relatively low viscosity in order to be easily pumped. In addition, the mud must have an acceptable gel strength in order to suspend the solid material if circulation is interrupted and to prevent accumulation of solids at the bit thus preventing mechanical jamming. The muds must also provide sufficient equalizing pressure to contain any liquid or gaseous components encountered in the natural formations pierced by the drill. By using proper mud formulations, possible collapses or blowouts resulting from uncontrolled influxes of formation fluids may be prevented. Inorganic colloidal materials such as bentonitic clays are generally added to the mud formulation to impart a desired thixotropic or gel structure to the system.
In addition to preventing the flow of formation fluids into the hole, drilling muds should also minimize the flow of fluid out into the formation. Both functions are served by the ability of the mud to form a thin but substantially impervious filter cake against exposed permeable formations. Excessive loss of liquid to the formation results in increased mud viscosities which causes decreases in drilling times in addition to poorer cuttings removal and lubrication. In U.S. Pat. No. 2,650,905 (issued Sept. 1, 1953 to Fordyce et al), use of sulfonated polystyrene polymers employed as drilling mud additives was shown to aid in forming substantially impermeable films around the walls of the bore hole in order to decrease the loss of fluid from the mud to the surrounding earthen formation while at the same time not affecting the viscosity of the muds.
The efficiency of the drilling process is related to the velocity at which the mud travels up the annulus which in turn is related to its viscosity as well as flow and density properties. Drilling mud viscosity is known to depend on the quality, concentration and state of dispersion of the colloidal solids of the mud. As the drilling operation proceeds, the thixotropy of the mud may be deleteriously affected by such factors as the character of the drilled strata, the loss or gain of water to the mud, chemically active contaminants which may flocculate the mud, the pH of the mud, and especially the increasing temperatures and pressures encountered at deeper drilling depths. In order to maintain workable viscosities, the muds must be formulated to dynamically respond to varying circumstances and conditions encountered during use. Due to the fact that improvements in efficiency are realized as the viscosity and density of a mud are increased, it is desirable to optimize drilling mud formulations to possess the highest viscosity and density workably feasible for a given formation at a given depth. Oftentimes colloidal stabilizing materials are employed.
Various naturally occurring anionic polymers and their derivatives such as mined lignins, tannins, polyphosphates and lignosulfonates have been employed as thinners or dispersants in drilling operations to depths of up to about 12,000 feet. Unfortunately these materials are unstable under increased temperatures and pressures, sometimes exceeding 450.degree. F. and 5000 psi, which are encountered upon deeper penetration of drilling operations. Upon exposure to such adverse conditions, flow resistance of the muds increases due to dispersant degradation resulting in excessive gel strengths and solidification. More energy and higher pressures are then required to pump the muds. Moreover, the instability of these additives usually requires their near to complete replacement during mud reconditioning before recirculation into the drill hole which results in increased drilling operation costs due to significant lost drill time. For these reasons more costly oil-based muds are sometimes chosen to be employed in the field due to their ability to withstand repeated exposures to high temperatures and pressures.
U.S. Pat. No. 3,730,900 (issued May 1, 1973 to Perricone et al) describes the use of low molecular weight styrene sulfonic acid-maleic anhydride copolymers as drilling mud additives useful as colloidal stabilizers. The copolymers are described as being particularly effective when drilling under high temperature and pressure environments.
The subterranean well drilling industry is still searching for means which provide improved aqueous drilling muds which are stable after prolonged as well as repeated exposure to high temperatures and pressures. For instance, the ability to extend the stability of a drilling mud subjected to a temperature of 500.degree. F. by a period of as little as 0.5 to 0.1 hours would be recognized in the field as a significant improvement. In addition, there is also a need in the art for a drilling mud additive which has increased tolerance to salt and/or highly alkaline pH environments.
The present invention provides drilling mud additives with improved high temperature-high pressure stabilities over styrene-sulfonic acid maleic anhydride copolymer dispersants. By employing the additives described herein in drilling mud formulations, superior performance and efficiency in drilling operations may be realized due to fewer stuck pipes resulting in fewer shutdowns, less energy required to pump the mud, as well as the ability to more efficiently reuse the additives after repeated exposure to high temperature-high pressure conditions. The additives also possess improved salt and high alkaline pH tolerance.